The application range of networks configured by the Ethernet (Ethernet is a registered commercial trademark.) is expanding and is starting to be applied to storage area networks (SAN). Moreover, data transfer is performed at increasingly high speeds by data transfer standards such as Gigabit Ethernet and 10 Gigabit Ethernet, etc. However, the problem of attaining an effective data transfer speed has arisen due to limits on protocols (hereafter TCP, etc.) such as TCP and SCTP that require an acknowledgment.
Operating specifications such as for TCP are described in non-patent document 2 or in non-patent document 5. The operation for establishing a TCP connection and the data transfer status operation are described here.
The exchange method for TCP control information and packet exchange in the TCP connections among communication terminals conforming to TCP are shown in FIG. 3. An example where a server 201 and a storage device 221 are communicating is described.
The server 201 sends a packet (SYN packet) set with SYN flags for TCP, to the storage device 221 as a connection request.
The storage device 221 receives the SYN packet from the server as an acknowledgment, and sends a packet (ACK packet) set with an ACK flag, to the server 201. The storage device 221 simultaneously sets a SYN flag in that ACK packet, as a connection request from the storage device 221 to the server 201. The server 201 here sends the ACK packet to the storage device 221 as an acknowledgment confirming that SYN packet from the storage device 221 was received, and establishes a TCP connection.
The server 201 and the storage device 221 shift to data transfer state when the TCP connection is established. In this data state, the transmit sections of the communication terminals send data to each other, and the receive sections utilize ACK packets as an acknowledgment confirming arrival of the data. For the purpose of simplicity, only the sending of data from the storage device 221 to the server 201 is described in the example related here.
In communication with TCP, the side sending the data (in this case, storage device 221) must standby until an acknowledgment is received, before it can send the next data. Therefore the round-trip time (RTT) which is the difference between the data transmission time and acknowledgment arrival time, is a factor in determining the upper threshold of transmit speed for the data to be sent. The quantity of data that can be transmitted in advance of the arrival of the acknowledgment is called the window. This window is also a factor in determining the upper threshold of transmit speed for data to be sent.
In the technology of the related art that adjusts these parameters to improve the data transfer throughput of TCP, a device is installed for relaying the connection such as TCP between the transmit terminal and the receive terminal. This device functions as a proxy for sending the acknowledgment, without waiting for an acknowledgment from the receive side, so that the RTT can be shortened and the data transfer throughput between the terminals can be improved. This technology for example, is the packet relay device disclosed in the patent document 1 and, the TCP overlay networks disclosed in non-patent document 3 and non-patent document 4.
The device for relaying the TCP connection is installed as a switch or router on the network. The acknowledgment performed by proxy by the switch 211 is described using FIG. 4. When the TCP connection is established and the data transfer state attained, the switch 211 acting as a proxy for the server 201, sends an acknowledgment for data sent to the server 201 from the storage device 221. In this case, the storage device 221 can send data in a quantity complying with the window, without waiting for an acknowledgment from the server 201. The RTT and window are here the same as in the case of FIG. 3, however the data transfer throughput of the TCP possesses a better data transfer throughput.
The packet transfer device serving as a proxy for sending the acknowledgment to confirm the connection such as TCP is hereafter called the proxy device for TCP.    [Patent document 1] JP-A No. 32295/2003    [Non-patent document 1] J. Poste, “Transmission Control Protocol”, Request for Comments 793, 1981    [Non-patent document 2] W. Stevens, “TCP/IP Illustrated, Volume 1: The Protocols”, Addison-Wesley, 1994    [Non-patent document 3] Tsutomu Murase, Hideyuki Shimonishi, Yohei Hasegawa, “Proposal for TCP Overlay Network”, Society of IEEE general meeting, 2002.    [Non-patent document 4] Ichinoshin Maki, Takeshi Hasegawa, Masayuki Murata Tsutomu Murase, “Performance Analysis of TCP Connection Segment Mechanisms on TCP Overlay Networks”, IEEE, IN03-198, 2004.    [Non-patent document 5] R. Stewart, Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer, T. Taylor, I. Rytina, M. Kalla, L. Zhang V. Paxon, “Stream Control Transmission Protocol”, Request for Comments 2960, 2000.